The data format of high speed one-bit audio signals which are modulated in the ΔΣ manner has an extremely high sampling frequency of 64 times 44.1 kHz and a short data word length of one bit, as compared with that of the conventional digital audio signals which has a sampling frequency of 44.1 kHz and a data word length of 16 bits. Thus, by employing the high speed one-bit audio signals, a broad transmittable frequency band can be obtained. Even though the data word of the high speed one-bit audio signals modulated in the ΔΣ manner is one-bit, by employing the signals, a high dynamic range can be secured in the audio band which is low against the over sampling frequency of 64 times the sampling frequency. These characteristics can be applied to data recording and data transmission maintaining high tone quality.
The ΔΣ modulation itself is not a new technique, and conventionally the ΔΣ modulating circuit is often used in an A/D converter, since the circuit can suitably be configured in the form of an IC, and A/D conversion with high accuracy can be performed without difficulty.
The ΔΣ-modulated signals or signals which are modulated in the ΔΣ manner can be restored to original analog signals by causing the ΔΣ-modulated signals to pass through an analog low-pass filter of simple configuration.
In transmitting the one-bit ΔΣ-modulated audio signals, there arise radiation noises due to a transmission line or a transmission circuit, and the generated radiation noises break into an analog audio unit and has a bad effect thereon. In order to solve the problem, the applicant of the present invention proposed a phase modulating method in “Signal Transmitting Apparatus and Method” disclosed in Japanese Laying-Open Patent H-9-186728, in which transmission signals and signals of inverted phases or signals whose phases are opposite to those of the transmission signals are alternately transmitted at a double transmission rate. In employing the phase modulating method, analog audio signals which are sufficiently low against the transmission rate are canceled by those of inverted phases, and the analog audio signals are sufficiently suppressed to low level.
When transmitting the one-bit ΔΣ-modulated audio signals, a transmitting method is desired which can transmit music signals as original signals as well as information data such as data character.
When transmitting the music signals and information data added thereto, the transmission rate is desired to be integer multiple of that of the music signals due to the facility of the processing. Thus, data to be transmitted becomes at least double in amount, and finally a quadruple transmission rate is undesirably required since signals of inverted phases are added thereto.